This invention relates to a retroreflective photoelectric sensor which may be used with a reflector to together sandwich a target detection area for detecting a light-reflecting target object in the detection area.
A retroreflective photoelectric sensor is used generally with a reflector to together sandwich a target detection area therebetween and detects the presence or absence of a target object of detection based on the difference in the character of reflected light from the reflector and that from the target object of detection for light emitted from the sensor. In the case of a retroreflective photoelectric sensor for a target object, a reflector with a reflection characteristic that affects the polarization mode of the light emitted from the sensor is used, and the presence or absence of a target object of detection is determined on the basis of whether or not a polarization component peculiar to the reflected light from the reflector is contained in the light received from the target area of detection. For this purpose, it has been known to use reflectors of the type, for example, having many triangular pyramid-shaped indentations distributed on the reflective surface such that the incident light is reflected several times on the three surfaces around the top point of the pyramid shape to convert the polarization mode of the incident light which is initially linearly polarized and to return the reflected light back in the direction from which the incident light came.
The present inventors have earlier proposed a retroreflective photoelectric sensor of a so-called biaxial kind. For this sensor, a light-emitting lens and a light-receiving lens with small polarization distortion with retardation value less than 17 nm, made by injection-molding of a resin material, were used, and an optical system for the light emission was made by arranging a light-emitting element, a first polarizer (say, for vertical polarization) and the light-emitting lens in this order, another optical system for the light reception being made by arranging the light-receiving lens, a second polarizer (say, for horizontal polarization) and a light-receiving element, in this order.
Similarly, the present inventors also proposed (in Japanese Patent Publication Tokkai 2001-228260) a different retroreflective photoelectric sensor of a so-called coaxial kind, comprising a light-emitting optical system for emitting light from a light-emitting element by passing it through a first polarizer (say, for vertical polarization), a light-receiving optical system for receiving light through a second polarizer (say, for horizontal polarization) and converting the received light into an electrical signal by using a light-receiving element, a single common lens for both emitting light from the light-emitting element and receiving light to be received by the light-receiving element and a beam splitter placed between the light-emitting and light-receiving optical systems and the common lens for directing both the outgoing light from the light-emitting element to the common lens and the incoming light through the common lens to the light-receiving optical system.
A sensor with this structure is advantageous in that the first and second polarizers, which used to be placed in front of the respective lenses according to the earlier technology, are now placed behind the lens and they may be made smaller in size and hence that the sensor case and the lens can be integrally formed, thereby contributing to significantly reduce the production cost of the sensor.
With a sensor of this structure with both the first and second polarizers placed behind the lens, however, there was a problem of light leakage even if the two polarizers are set in the mutually perpendicular relationship (or the cross-nicol relationship, forming so-called crossed nicols) such that the quantity of received light in the presence of a light-reflecting target object could not be made sufficiently smaller than that in its absence.